In recent years, urea-SCR (selective reduction) systems have been developed in electric power plants, various factories, vehicles, and the like. Particularly, in the field of vehicles (especially, a diesel engine vehicle), post treatment techniques of exhaust gas for purifying and reducing NOx (nitrogen oxides) in the exhaust gas are classified into two important trends, namely, the above-described urea-SCR system, and a NOx storage-reduction catalyst. The urea-SCR system is already put into practical use in large trucks, and known to have a high purification ratio of a maximum of about “90%”. Presently, the general urea-SCR systems which are now studied for application to diesel engines are designed to reduce (purify) NOx in the exhaust gas by means of NH3 (ammonia) generated from a urea ((NH2)2CO) aqueous solution (hereinafter referred to as a urea water).
Conventionally, the system disclosed in JP-A-2003-293739 is known as a specific example of such a urea-SCR system. This system mainly includes a catalyst for promoting a specific exhaust gas purifying reaction (reduction reaction of NOx), an exhaust pipe for guiding the exhaust gas discharged from an exhaust gas generating source (for example, an internal combustion engine) to the catalyst, and a urea water addition valve disposed at a midway point of the exhaust pipe for injecting and adding the urea aqueous solution (additive) to the exhaust gas flowing in the exhaust pipe. The system with this arrangement is configured to inject and add the urea aqueous solution into the exhaust gas by the urea water addition valve, and to supply the urea aqueous solution to the catalyst on the downstream side together with the exhaust gas, using a flow of the exhaust gas. The urea aqueous solution thus supplied is hydrolyzed by exhaust gas heat or the like to generate NH3 (ammonia), as represented by the following chemical equation: (NH2)2CO+H2O→2NH3+CO2. This leads to a reduction reaction of NOx by the NH3 on the catalyst, through which the exhaust gas is purified.
However, the catalyst used in such purification of the exhaust gas generally promotes the reduction reaction of NOx in a temperature range exceeding an activation temperature (critical reaction temperature) inherent to the catalyst, that is, a temperature range having the activation temperature as the lower limit. Thus, the system as disclosed in JP-A-2003-293739 cannot have a sufficient capacity of purifying the exhaust gas when the catalyst is at a low temperature below the activation temperature.
Most of general catalysts for purification of exhaust gas for use in, for example, a vehicle-mounted internal combustion engine or the like have the activation temperature of about “180° C.”. In contrast, the temperature of the exhaust gas emitted from the internal combustion engine during idling is generally about “140 to 150° C.”. Thus, when the internal combustion engine (engine) serving as the exhaust gas generating source starts to accelerate from the idling state, the sufficient exhaust gas purification capacity is not obtained even though the increase in amount of emission of NOx is predicted due to a high load operation. This may lead to deterioration of exhaust emission characteristics. The same kind of problem may also be posed at other times, including startup of the engine.